Hybrid powertrains incorporating electrically-variable transmissions (EVT) have been proposed by many sources. The typical hybrid powertrain includes an internal combustion engine and a plurality of electrical motor/generators, the power output of which are combined through planetary gearing to produce an output drive for the vehicle powertrain.
The electrical units are capable of operating as electric motors or as generators. In the motor phase, they of course provide power to the transmission, and in the generator phase, they take power from the transmission. In many of the hybrid powertrains incorporating an EVT, at least one and generally two pure mechanical points are arrived at during the operation of the powertrain. At a mechanical power point, at least one of the electrical units is at zero speed, thus providing a ground mechanism for a portion of the planetary transmission.
When the electrical unit is at zero speed, it is generally operating as a brake or a retarding device and therefore still requires energy to maintain sufficient torque to prevent the components connected therewith from rotating. This electrical energy, which is distributed to the unit, produces heat within the transmission and must be dispersed through the cooling system. While the overall efficiency of the powertrain at the mechanical point is improved, it could be more greatly improved if the electrical unit was not absorbing the energy required for braking.